From: Simon Glass sjg@chromium.org Date: Wed, 25 Aug 2021 21:15:00 -0600
Hi Heinrich,
On Wed, 25 Aug 2021 at 02:05, Heinrich Schuchardt xypron.glpk@gmx.de wrote:
Hello Simon,
some boards like qemu-riscv64_defconfig do not use any device-tree at build time. A device-tree is only supplied at runtime by the prior boot stage (CONFIG_OF_PRIOR_STAGE=y).
In doc/develop/devicetree/intro.rst you suggest to put binary blobs into the device-tree.
Could you, please, update the documentation to explain how adding blobs to the device-tree works in the different scenarios depending on the values of:
CONFIG_OF_EMBED CONFIG_OF_SEPARATE CONFIG_OF_BOARD CONFIG_OF_HOSTFILE CONFIG_OF_PRIOR_STAGE
It would be especially important to understand how one can develop a board independent feature which works for all of these settings.
OK I will take a crack at this tomorrow.
But I think there is a disconnect here, since the only options that matter within U-Boot are OF_SEPARATE and OF_HOSTFILE, which both use a u-boot.dtb file. There is nothing tricky here.
The OF_BOARD business is for when the board does special things. Presumably signing will do special things too. We cannot really know what those things are because the board as 'opted out' of the standard options.
Please, describe CONFIG_OF_PRIOR_STAGE in doc/develop/devicetree/control.rst.
So I'm not allowed to delete that option? :-) It seems to me to be extremely sparse on documentation. We need an explanation of what it means and what effect it has on the build system, U-Boot and some discussion of how qemu works. It seems to have been added as part of an unrelated broadcom commit. The tags were incorrect so I doubt anyone noticed it. Since then it has apparently proved useful elsewhere, but no one has added more docs.
So perhaps you can help me with my doc by explaining how OF_PRIOR_STAGE works in qmeu, why the DT in U-Boot cannot be used, and which project actually hosts the DT that qemu provides? Armed with that information, I might be able to make sense of it all.
Well, QEMU allows configuration of (emulated/paravirtualised) devices from the command line. So it is pretty much impossible for U-Boot to provide a DT that matches that configuration without adding a lot of code the dynamically build it from some sort of configuration specification provided by QEMU to U-Boot. But at that point QEMU might as well provide the DT itself, don't you agree?
Anyway, replying to this thread since for the Apple M1 support that I'm working on we're in a somewhat similar situation. The Asahi Linux project has implemented m1n1 as a bootloader and plans to use U-Boot as a "payload" to implement booting a standard Linux distribution (and other OSes). In this scenario m1n1 actually provides the DT since it parses Apple's version of the DT (which isn't in the standard DT format) and adds/updates certain properties that depend on the actual hardware it is running on.
For this code I use CONFIG_OF_BOARD and implement board_fdt_blob_setup() to simple return a pointer to the DT that m1n1 set up. But that seems to be exactly what CONFIG_OF_PRIOR_STAGE does...